1. Field of the Invention
The present invention relates to a high-frequency power amplifier and, more particularly, to a high-frequency power amplifier used, for example, in apparatuses for mobile communication and satellite communication in microwave and milliwave bands, for example, from several megahertz to several hundred gigahertz.
2. Description of the Related Art
In recent years, there has been an increasing demand for communication apparatuses of a smaller size and higher output to be used in microwave and milliwave bands. There has also been an increasing demand for improving the quality of propagated signals. With these demands, a demand has arisen for a high-frequency power amplifier having low-distortion characteristics and having a high efficiency.
In particular, in a microwave communication system using a multicarrier signal or a modulated wave signal according to the code division multiple access (CDMA) system in recent years, an amplifier for signal amplification is used by being operated at an output level much lower than the maximum power such as to avoid the influence of a distortion due to nonlinearity of the amplifier.
In high-output power amplifiers used in ground stations for mobile communication or satellite communication, there is a need to use a transistor element having a large gate width in order to perform high-power amplification. To form such a transistor element, a multiplicity of gate-finger-type transistors are combined by being connected in parallel with each other. In a case where a large-gate-width transistor element is formed by connecting a multiplicity of gate-finger-type transistors connected in parallel with each other, the input and output impedances of the transistor element are necessarily decreased and a matching circuit load for impedance matching to the transistor at the operating frequency of the transistor is extremely small.
It is known that a high-frequency amplifier operates at high efficiency if a load condition close to a short-circuited condition is set with respect to the second harmonic on the input side. However, to provide a load condition close to a short-circuited condition with respect to the second harmonic of the operating frequency for the purpose of operating a large-gate-width transistor element such as high-frequency power amplifier at high efficiency, in addition to setting a matching circuit load of an extremely low impedance with respect to the fundamental of the operating frequency, setting the impedance of the matching circuit load to a further low value with respect to the harmonic of the operating frequency is required.
As a known high-frequency power amplifier, an arrangement has been disclosed in which a circuit formed of a second line having an electrical length substantially equal to the ¼ wavelength with respect to a fundamental frequency and a capacitor are connected in series is provided between a grounding end and a connection point B existing between the gate of a power transistor and an input matching line. This circuit ensures that the impedance of the second line as seen from the point B is substantially zero and the input impedance with respect to the second harmonic can be controlled without influencing the input impedance with respect to the fundamental wave, thereby improving the operation efficiency (see, for example, Japanese Patent Laid-Open No. 8-37433, paragraph No. [0039] and FIG. 2).
As another known high-frequency power amplifier, an arrangement has been disclosed in which GaAs field effect transistors (FETs) formed of semiconductor chips and provided as power amplification devices are respectively disposed in a source grounded state in a first path A and a second path B between an input terminal IN and an output terminal OUT, and in which input and output matching circuits each including a resonance circuit as an equivalent circuit are respectively provided on the input and output sides of each GaAs FET (see, for example, Japanese Patent Laid-Open No. 2005-109651, paragraph No. [0006] and FIG. 1).
Also, as still another known broad-band power amplifier, an arrangement has been disclosed in which band-pass filters having predetermined passbands for determining frequency amplification characteristics are disposed in stages before and after each of three power amplifiers. By disposing these band-pass filters, the three power amplifiers are arranged as narrow-band power amplifiers having such frequency amplification characteristics as to perform amplification in three successive frequency bands (see, for example, Japanese Patent Laid-Open No. 2002-43873, paragraph No. [0006] and FIGS. 1 and 2).
In the power amplifiers described in the above-mentioned patent documents, however, a circuit for controlling harmonics, a resonance circuit or a band-pass filter is disposed on the external circuit side of a transistor element combined as one power transistor or one chip with input and output terminals.
In the case of control of harmonics of a large-gate-width transistor element such as a high-frequency power amplifier, there was a problem that it is difficult to obtain a low-impedance load with respect to harmonics even if a processing circuit for controlling harmonics is added on the external circuit side of each transistor element whose input and output ends are respectively combined as used to be. Further, if the transistor element has an operation frequency bandwidth, second or higher harmonic frequencies are generated in a certain bandwidth as well as operating frequencies and there is a need to control the input load throughout a wide band.